The present invention relates to ultrasonic transducers for use with devices using high frequency acoustic radiation and more particularly to such transducers which are suitable for use in acoustic microscopes.
Recent advances in the generation and detection of high frequency acoustic waves extending up to 1 GHz have made possible an acoustic wave length of about 1 micron under water, giving rise to the availability of an acoustic microscope.
More particularly, an acoustic wave beam of an extremely small size is produced which is projected on a target specimen and the propagation loss of acoustic radiation due to reflection, scattering and penetrant attenuation at the target is detected to obtain information representative of the elastic properties of the target. In order to apply this principle to an acoustic microscope, a surface of the specimen is scanned two-dimensionally with the focused acoustic wave beam and the perturbed energy is displayed on a cathode-ray tube in synchronism with the scanning.
In such an apparatus, the resolution which is a fundamental characteristic of this type of apparatus depends on the extent to which the size of the acoustic wave beam is reduced. A prior art ultrasonic transducer, as shown in FIG. 1, directed to such a reduction in beam size has a cylindrical crystalline body 20 as an ultrasonic wave propagation medium of sapphire, for example, with one flat surface optically polished and an opposite surface formed with a concaved recess 25. An RF electric signal produced from an electric signal source 10 is applied to a piezoelectric film 15 which in turn transmits an RF acoustic wave in the form of a plane wave into the crystalline body 20. The acoustic plane wave is focused at a given focal point F by means of a positive acoustical lens 40 formed at an interface between the arcuate recess 25 and an ultrasonic wave focusing medium 30, typically water. As well known in the art, a sufficiently small ratio between focal length and aperture size, that is, a sufficiently small F-number of the lens can contribute to generation of the ultrasonic wave beam of a small size which approximates its wave length. When irradiating this beam onto a target, perturbed ultrasonic energy is produced from the target. For reception of the perturbed energy, it is possible to employ either a reflection mode using the same crystalline body and piezoelectric film shown in FIG. 1 or a transmission mode using a crystalline body and a piezoelectric element, similar to those of FIG. 1, which are positioned confocally.
Let R, C.sub.1 and C.sub.2 denote a radius of curvature of the concaved ultrasonic lens 40, the speed of sound in the lens material and the speed of sound in the focusing medium, respectively. Then, a front-face focal length F is, ##EQU1## and a back-face focal length F' is, EQU F'=R(C.sub.1 /C.sub.2) (2)
The lens effect can be determined by multiplying a sound pressure distribution on the back-face focal plane by a pupil function of the lens and subjecting the product to a two-dimensional Hankel transformation. According to a lens theory in optics, for the sake of obtaining good focussing effect, it is required that the sound pressure distribution lie on the back-face focal plane and that the sound pressure distribution on the back-face focal plane be of a uniform amplitude and phase of a plane wave or subject to a Gaussian distribution in respect of amplitude and phase of a plane wave. Another amplitude distribution may also attain the focussing effect but it requires a great number of multi-lens systems for elimination of the lens aberration and is impractical for industrial purposes.
When the piezoelectric film shown in FIG. 1 is driven, the sound pressure distribution occurs on the back-face focal plane inside the lens and assumes a sophisticated pattern under the influence of the interference of acoustic wave. Therefore, it is of a great significance in lens design to select aperture size (diameter) 2.rho..sub.o of the piezoelectric film, distance l between the film and the back-face focal plane of the lens, and aperture size 2a of the lens.
Various sound pressure distributions of the acoustic wave transmitted from the piezoelectric film to the interior of the lens are graphically shown in FIG. 2 by using the above values. In the figure, a curve on the left of the ordinate axis represents a sound pressure distribution along the lens axis and curves on the right represent orientational distributions at distances in terms of normalized l by .rho..sub.o.sup.2 /.lambda., .lambda. being the wavelength of acoustic wave used. It will be appreciated that within a distance of 1 (one) or .rho..sub.o.sup.2 /.lambda. from the piezoelectric film covering a so-called near field, sophisticated patterns occur which are due to the interference of the acoustic wave whereas outside the distance of 1 or in a so-called far field, a Gaussian-like (strictly, Airy function) distribution occurs. Here, .rho..sub.o.sup.2 /.lambda. is usually called a Fresnel focal distance.
Therefore, in a first prior art lens design, .rho..sub.o, l and a are so designed as to yield the far field sound pressure distribution on the back-face focal plane of the lens by determining l=.rho..sub.o.sup.2 /.lambda. and a.perspectiveto..rho..sub.o. Thus, as will be seen from FIG. 2, the acoustic wave obviously assumes the Gaussian-like sound pressure distribution on the back-face focal plane. More specifically, as shown in FIG. 3, the acoustic wave which is expected to assume the sound pressure distribution at point A.sub.o (corresponding to point B in FIG. 2) which is distant from the piezoelectric film by .rho..sub.o.sup.2 /.lambda. is irradiated onto the lens having an aperture of 2a (=2.rho..sub.o).
Pursuant to a second lens design, the distance between the back-face focal plane of the lens and the piezoelectric film is reduced to an extent that no interference of ultrasonic wave occurs. While this second design has many applications in the range of MHz frequencies, it is almost impractical in the range of GHz frequencies. Because with sapphire as a lens material, the ultrasonic wave at 1 GHz has a wavelength of about 11 .mu.m and there needs preparation of an extremely thin lens. Therefore, the first prior art lens design alone is practical.
The arrangement according to the first prior art lens design, however, is disadvantageous as will be described below.
In the first place, as the frequency increases, the Fresnel focal distance .rho..sub.o.sup.2 /.lambda. increases accordingly, a disadvantage thereby being that ultrasonic attenuation in the crystalline body forming the lens is aggravated and the cost for material is increased. For .rho..sub.o being 1 mm, for example, .rho..sub.o.sup.2 /.lambda. for sapphire is drastically prolonged, amounting to about 91 mm with an accompanied attenuation of 5 dB. For a fused silica lens, .rho..sub.o.sup.2 /.lambda. is 166 mm and the attenuation is 54 dB.
In the second place, when the acoustic wave is necessarily increased in frequency to increase the resolution of the acoustic microscope, it suffers from a large attenuation within the focusing medium (typically water) in which it is focused. Accordingly, in order to obtain a high resolution, a lens is needed having a small aperture. Reduction in lens aperture corresponds to reduction in .rho..sub.o.sup.2 /.lambda. so that in compliance with the reduced lens aperture, it is necessary to prepare a piezoelectric film of a reduced diameter of the same size. For 1 GHz, for example, the desirable lens aperture is 100 .mu.m but a piezoelectric film of the corresponding 100 .mu.m aperture is difficult to prepare and to handle and in addition, has a high impedance level for which the impedance matching is difficult at RF electric signal supplied.
As described above, the prior art has many difficulties for production of an ultrasonic transducer since it requires an extensively elongated crystalline body and a piezoelectric film of a reduced diameter of the same size as the reduced lens aperture.